To ensure adhesion within an insulating layer and an inner layer circuits in conventional multi-layered printed wiring boards (hereinafter referred to as “MLB”), fine copper oxide particles (a so-called “black oxide treatment”, and hereinafter referred to as “a B/O treatment”) have been made on it as disclosed in Patent Document 1. The reason for requirement on such a B/O treatment is to prevent a de-lamination between the shiny side of a copper foil constituting the inner layer circuit and the inner layer insulating material when a MLB undergoes thermal shock in a solder re-flow process or the like.
However, such a conventional B/O treatment is made of copper oxide particles, and so such portions can be etched more rapidly than portions of copper metal. Consequently, etching after such a B/O treatment results removal of the B/O treatment on the circuit edge. In such a way, so called “haloing” are caused in the fringe of the inner layer circuit patterns; thus, when a PWB with such an inner layer circuit, the peripheries of the land portions shows pink colored rings through substrate, which has been sometimes referred to as “pink rings.”
So, a process for solving the problems in such a traditional B/O treatment has come to be generally used. In which, copper oxide is once formed on the surface of the inner layer circuit, and then the copper oxide is subjected to a reduction treatment to convert the surface to be a copper metal as referred in Patent Documents 2 to 4. This method can prevent the generation of the above described haloing phenomena.    Patent Document 1: Japanese Patent Laid-Open No. 64-37081    Patent Document 2: Japanese Patent Laid-Open No. 3-87092    Patent Document 3: Japanese Patent Laid-Open No. 4-217391    Patent Document 4: Japanese Patent Laid-Open No. 5-152740